External scripts in safe mode

Hello If this is an inappropriate place to seek help, many apologies, and where might I try? I have a PHP 4.1.2 script on Red Hat (Apache 1.3.27) which calls an external script (Tidy, from tidy.sourceforge.net). It is called via popen() so that I can pipe through some arguments and receive the output. This worked fine until my ISP turned on safe mode. Now, although the UID of both the calling PHP script and of Tidy are the same, and though Tidy is sitting in a directory specified in safe_mode_exec_dir, the script fails. Is there anything else I should be checking? Thanks Tom Chadwin This email and its attachments are intended for the addressee only and may be confidential or the subject of legal privilege. If this email and its attachments have come to you in error, you must take no action based on them, nor must you copy them, distribute them or show them to anyone. Please contact the sender to notify us of the error. Durham City Council accept no liability for, and shall not be legally bound by, the contents of this email. No assurances are given as to the authority of the sender to act on the Council's behalf and the views expressed within this email may not be official Council policy but instead the personal views of the distributor. This email and any attached files have been scanned for the presence of computer viruses however, you are advised that Durham City Council bears no liability for attachments and you therefore open any attachments at your own risk.